Synthesis of cyclopentene-fused pyrroloisoquinolinone derivative via N-acyliminium ion cyclization

In this research, a new class of pyrrolo[2,1-a]isoquinolinone derivative 7 was prepared. C-4 Hydroxylated 5-ethoxylactam 1b gave 4,5-epoxylactam 2 in high yield instead of isoquinoline derivative 3b under the N-acyliminium ion cyclization condition. The 4,5-epoxylactam 2 was converted to another N-acyliminium ion precursor 6 through base-promoted epoxide ring opening, silylation, and thermal Diels-Alder reaction with cyclopentadiene in high yield. Finally, compound 6 was cyclized in the presence of TiCl₄ to provide cyclopentene-fused pyrroloisoquinoline derivative 7.